This application claims priority of European Patent Application No. 98303418.2, which was filed on April 30, 1998.
This invention relates to channel and data estimation methods and apparatus in mobile radio communications and in particular to an equalizer that compensates for channel distortion by iterating data and channel estimation procedures on a block-by-block basis.
In digital mobile radio communications, transmission channels suffer from severe distortion due to frequency selective fading. In addition, channel characteristics are normally time-varying due to the relative motion of fixed and mobile stations. Therefore, in order to allow for reliable transmission, the receiver must be able to estimate and compensate for channel distortion on a block-by-block basis. Various channel estimation and channel equalization methods have been proposed in literature and are commonly used in practical systems such as mobile cellular communication systems employing the European wireless digital cellular standard xe2x80x9cGSMxe2x80x2. In most cases the receiver performs channel equalization on the received signal using Maximum Likelihood (ML) or Maximum A Posteriori (MAP) probability data estimation, based on the knowledge of the Channel Impulse Response (CIR). Most practical systems employ training sequences to enable the CIR to be estimated before the equalizer start-up. Fast time varying, fading channels require the changing channel response to be tracked and adjusted dynamically by the receiver for the duration of the received signal. Tracking of the CIR may be performed by means of decision directed algorithms, where tentative decisions from the equalizer are used to update the initial CIR estimate. Examples of receiver systems which perform channel estimation and channel equalization may be found in the following articles: xe2x80x9cBit Synchronization and Timing Sensitivity in Adaptive Viterbi Equalizers for Narrowband TDMA Digital Mobile Radio 25 Systemsxe2x80x9d, A. Baler, G. Heinrich and U. Wellens, Proc. IEEE Vehicular Technology Conference, June 1988, pp 377-384; xe2x80x9cCorrelative and Iterative Channel Estimation in Adaptive Viterbi Equalizers for TDMA Mobile Radioxe2x80x9d, ITG-Fachbericht No. 107, VDE Verlag, April 1989, pp 363-368; xe2x80x9cSimulation and Hardware Implementation of a Viterbi Equalizer for the GSM TDMA Digital Mobile Radio Systemxe2x80x9d, A. Baler, G. Heinrich, P. Shoeffel and W. Stahl, Proc. 3""d Nordic Seminar on Digital Land Mobile Radio Communications, September 1988 pp. 13.7.1. 13.7.5.
The effectiveness of the channel estimation strategy, and thus the overall equalization performance, depends heavily on the reliability of the initial CIR estimate. There is a requirement for an improved estimation strategy that can function with or without training sequences.
According to a first aspect of the invention there is provided a method of estimating channel impulse response and data in a signal transmitted over a channel in a communication system. The method estimates the channel impulse response using correlative channel sounding. The estimated channel impulse response is used to estimate the data in the signal, and an output is provided. The channel impulse response estimating step is repeated, at least once, using the previous output and an improved channel impulse response estimate is provided, for use in a repeated data estimating step.
According to a second aspect of the invention there is provided apparatus for estimating channel impulse response and data in a signal transmitted over a channel in a communication system. The apparatus has a channel impulse response estimator for providing an initial channel impulse response estimate using correlative channel sounding, a data estimator for providing an initial estimate of data in the transmitted signal, and an output. The channel impulse response estimator has a first input for receiving the transmitted signal, a second input for receiving a feedback signal from the apparatus output, and an output. The data estimator has a first input for receiving the channel impulse response estimate, a second input for receiving the transmitted signal, and an output.
The invention provides a reduced-noise CIR estimate, which is needed for the equalization of the received signal, in the case of multipath propagation environment, and thus provides improved receiver performance.
The equalizer performance is improved considerably by iterating the data and channel estimation procedure on a block-by-block basis. In particular, after a first pass in which the initial channel estimate is obtained by resorting to the known training bits, one or more iteration can be performed, using data symbol decisions from the equalizer output derived in the previous iteration, together with the original training sequence, to obtain a new CIR estimate.
In systems in which the information bits are encoded (and possibly interleaved) prior to modulation, the above strategy can be further improved by using the more reliable decisions obtained by a) re-encoding (and possibly re-interleaving) the channel decoder output, or b) simply taking (and possibly re-interleaving) the most significant bit of the A Posteriori values for the coded bits provided by a soft-in/soft-out channel decoder. Computer simulations carried out for the particular case of the GSM TCWFS transmission scheme show that, as compared with the conventional channel estimation approach (that is, correlative channel sounding by training sequence), the invention provides a significant performance improvement even with just one iteration.
The invention may also be used in those cases where no training sequence is available and the data estimation is performed by starting with an arbitrary channel estimate.